The present invention relates to a liquid crystal display device for driving a liquid crystal by means of an in-plane-switching system to apply an electric field which is almost parallel to a substrate between a picture element electrode and counter electrode.
An active matrix type liquid crystal display device using an active component, such as a thin film transistor (TFT), is widely used as a display, having such characteristics as being thin and light weight, and having a high displaying quality. A liquid crystal display device of this type is constructed with a liquid crystal provided between an active matrix substrate having a TFT element and an opposite substrate, and an electric field is applied between picture element electrodes and a counter electrode so as to drive and control the liquid crystal, whereby light incident on the liquid crystal is modulated so as to form an image.
Further, in order to widen the viewing angle of a liquid crystal display device, a liquid crystal display device using an in-plane-switching system has been proposed, in which the picture element electrode and the counter electrode are formed on the active matrix substrate, and an electric field is applied in parallel to the surface of the substrate. Such a device is described, for example, in Proceedings of the 15th International Display Research Conference, p. 707 (1995) or Japanese Patent Laid-open 7-36058 Bretin. The in-plane-switching system has the advantage of being able to provide a wide visual angle, because the liquid crystal is controlled by an electric field applied in a substantially transverse direction relative to the viewing angle.
The conventional display device using in-plane-switching has a configuration in which metal films, formed under a protective insulation film, serve as the picture element electrodes and the counter electrodes thereto. However, since the electric field is applied to the liquid crystal through the protective insulation film, an electrical voltage loss occurs, with the result that the driving voltage becomes large, causing a problem in that the power consumption increases. Moreover, while it is effective to make the spacing between the picture element electrodes and the counter electrodes big in order to enlarge the aperture ratio, this requires the driving voltage to be even larger. Since the threshold driving voltage of the liquid crystal is restricted by the upper limit of the driving voltage, there arises a problem in that the selection of a liquid crystal which has a fast responsiveness is limited.
Such an increase in the power consumption caused by such an electrical voltage loss may be avoided by providing a contact hole which forms an aperture in the gating isolation film and by providing an electrode for driving the liquid crystal through this aperture, as described in Japanese Patent Laid-open No. issue 7-128683 bulletin. However, the usual metal electrode has a problem in that it is corroded by the liquid crystal composition, so that it is necessary to form a protective insulation film on top of the electrodes. Naturally, in this case, the electrical voltage loss which occurs due to the protective insulation film increases even more.